In spacecraft that are in the take-off or landing phase through the earth's atmosphere, or spacecraft exposed to extreme thermal stress in orbit around the earth, it is necessary that the generated heat is safely and reliably removed. Evaporation heat exchangers are employed for this purpose.
The basic principle of operation of such heat exchangers lies in that, the medium to be cooled, referred to herein as coolant, circulates in an active liquid circulating circuit for the heat removal, by bringing the coolant into heat transferring contact with a medium to be evaporated, which is stored in a supply container. The evaporated medium in the form of vapor is then discharged out of the spacecraft into the surroundings.
In order to optimally use the medium to be evaporated the evaporation must be as complete as possible, whereby it is important to ensure as good a thermal contact as possible for the heat transfer between the cooling liquid and the medium to be evaporated.
In a conventional heat exchanger described, for example, in German Patent Publication DE-PS 3,718,873, (Muschelknautz et al.), published on Nov. 10, 1988, the cooling liquid or rather coolant flows through individual channels extending through a processing or heat exchange space, into which the medium to be evaporated is sprayed in droplet form through an inlet valve. In a second conventional evaporation heat exchanger, the cooling liquid or coolant flows openly through the processing space, while the medium to be evaporated is caused to flow through individual channels usually arranged in bundles passing through the heat exchange space. The cooling liquid is, in addition, forced into a meandering flow path through screens arranged in the processing space.
An evaporation heat exchanger of the immediately aforementioned type is described in an article: "HERMES Thermal Control Design And Architecture", by M. Bottacini, A. Moscatelli, and C. Ferro, 21st ICES-Conference, 1991, SAE 911 499, p. 12. This conventional heat exchanger requires that, independent of the heat load causing thermal stress to be removed, which in the case described in the above article can vary between 30 and 100% of the maximum load, the discharge temperature of the coolant, e.g. water, at the exit of the coolant circulating circuit is constant at 6.degree. C.
The liquid to be evaporated in said conventional heat exchanger is liquid ammonia (NH.sub.3), that is led from a respective supply container through a spraying system into the evaporator. After evaporation the vapor is blown out into the surroundings. The temperature of the ammonia in the supply container has a temperature between 0.degree. C. and 70.degree. C. and the pressure in the supply container corresponds to either the saturation pressure or it is increased by pressurizing the tank with nitrogen or helium gas.
Upon injection of the liquid ammonia into the evaporator, the pressure in the supply tank is drastically reduced. Therefore, directly behind the injection valve, as much ammonia is evaporated as is necessary for bringing the injected liquid to the saturation temperature prevailing downstream of the valve. This temperature is a function of the pressure in the evaporator.
Without special technical equipment, the pressure in the evaporator depends only on the following, namely the absolute pressure of the surroundings, into which the evaporated ammonia is released, the pressure loss of the mass flow of ammonia through the outlet channel, and on pressure jumps at ring cross-sections in the outlet channel.
The ammonia temperature that develops during the adiabatic evaporation must always be colder than the water temperature, so that heat from the coolant circulation circuit can be taken up. However, the evaporation temperature under any circumstances must not be so low that local freezing takes place in the surface or interface layer of the water current or flow.
Since such heat exchangers should work in a vacuum (at about 600 Pa) as well as at normal atmospheric pressure (101.3 kPa) and since various sizes of ammonia vapor streams or flows are generated, due to the wide load spectrum, the evaporation pressure, and therefore also the evaporation temperature vary depending on the load and on the mission to be performed. The evaporation temperature is the highest at a full load and the lowest at a partial load.
To regulate or control the ammonia evaporation pressure and the temperature in closed loop fashion, the above article by Bottacini et al. suggests providing an inlet pressure regulating valve on the inlet side where the medium to be evaporated enters the heat exchange system, whereby the pressure in the evaporation space or chamber can be held constant independently of the generated vapor mass flow and also independent of the pressure of the surroundings, e. g. in space.
Since such an inlet pressure regulating valve is a mechanical component, in which a failure can never be completely ruled out, it would be necessary to meet redundance requirements to provide at least one more valve. In fact, an additional parallel line with two more valves or three additional valves, would be needed. However, such an arrangement of a total of four valves means a substantial increase in expenditure, mass and volume, which is not desirable in a spacecraft.